Observations on the Mobility of the Silver-Studded Blue Plebejus Argus [Online]

30 October 2019 © Harry E. Clarke Citation: Clarke, H.E. (2019). Observations on the Mobility of the Silver-studded Blue Plebejus argus [Online]. Available from http://www.dispar.org/reference.php?id=150 [Accessed November 15, 2019]. Observations on the Mobility of the Silver-studded Blue Plebejus argus Harry E. Clarke Abstract: The Silver-studded Blue is often considered to be a sedentary species, not moving more than 50 m during its life. However, based on recent research and my own experience from Surrey, this is not the case. Males can easily move 200 m when relocated away from ant nests, and new sites can be colonised within a few years from source sites at least 3 km away, probably a year or so after the ants Lasius niger or Lasius alienus have colonised. Silver-studded Blue is a butterfly of early successional habitat, so it needs to be mobile in order to find suitable habitat for the next generation. If it was very sedentary, then colonies would soon die out as habitat matures and becomes unsuitable to support the ants and the butterfly. Introduction This article has been prompted by the statements that Silver-studded Blue (Plebejus argus) is sedentary, barely moving more than 20 m, with 50 m considered exceptional, and breeding populations of 100 m or more considered isolated, based on three mark-release-recapture studies (Thomas, 2014) and (Eeles, 2019). This does not agree with my own observations and does not seem consistent with the habitat requirements. Figure 1 - Male Silver-studded Blue on Bell Heather Photo © Harry E. Clarke The Silver-studded Blue is a butterfly of early successional habitat [1]. Unless something is keeping the habitat in check, early successional habitat develops into late successional habitat with heathland (one of the habitats occupied by the butterfly) degenerating into woodland. For an early successional butterfly to survive, it must be able to find new habitats. If it is very sedentary then it will soon die out, as the habitat becomes unsuitable. The Silver-studded Blue has a number of subspecies, indicating that some populations are isolated, with no interchange between them. The Silver- studded Blue forms discrete colonies and is certainly not as mobile as some species, such as the Brimstone (Gonepteryx rhamni), which roams over large areas. Ant Associations The Silver-studded Blue is one of the few obligate myrmecophiles [2] among the Palaeartic Lycaenidae, with entirely phytophagous (plant eating) non-inquiline [3] caterpillars (Fiedler, 2006). The Idas Blue (Plebejus idas) is the only other known European example (Fiedler, 1995). Obligate myrmecophiles are highly specific with regard to its ant hosts. Butterflies such as the Large Blue are inquiline social parasites of ant colonies (Fiedler, 2006). The subspecies argus of Silver-studded Blue on heathland has a proven relationship with Lasius niger (Fiedler, 2006; Wells, 2012; Seifert, 2018) and subspecies caernensis has a proven relationship with Lasius alienus (Harris, 2008; Seifert, 2018) on limestone sites. A proven relationship means that Silver-studded Blue larvae or pupae have been found in the ant nests, tended by the ants. 1 of 4 Figure 2 - Silver-studded Blue larva attended by Lasius niger Photo © Steve Ogden There is only circumstantial evidence for a relationshiop between the subspecies argus and Lasius platythorax. However, despite an extensive literature search and asking relevant experts, no proof of the relationship has been established. Wells (2012) found no relationship with Laisus psammophilus in Norfolk, despite the presence of both the ant and the butterfly. Habitat requirements The habitat requirements for the Silver-studded Blue are complex, since the ant, larval and adult requirements are all slightly different. However, for a successful colony to exist, these habitats must exist in close proximity. Figure 3 - Typical lowland heath Silver-studded Blue habitat at Silchester Common Photo © Peter Eeles Silver-studded Blue larvae require the sward height to be less than 7 cm (Thomas, 1985) and where the larval foodplant is growing. Recorded larval foodplants are Bell Heather (Erica cinerea), Common Bird's-foot Trefoil (Lotus corniculatus), Common Rock-rose (Helianthemum nummularium), Cross-leaved Heath (Erica tetralix), Gorse (Ulex europeaus), Ling (Calluna vulgaris) and Horseshoe Vetch (Hippocrepis comosa). Adults require a nectar source during the flight season. On heathland this is usually Bell Heather or Cross-leaved Heath. Ling does not flower until the end of the flight period. Adults also require somewhere to roost, which can be long grass, such as Purple Moor-grass (Molinia caerulea), mature heather or gorse (Ulex sp.), which can provide shelter from the wind. The habitat requirements for the ants are described in great detail in Seifert (2018). In brief, the habitat requirements for Lasius niger are that the nests are made in a mineral substrate (such as compact sand), in an unshaded location, on a flat or gentle slope. A slope that is too steep is probably too arid, which may explain the absence of the ants, and this is clearly demonstrated at Hankley Common in Surrey. Ant nests are created in bare ground and, as vegetation grows, the ants attempt to keep the roof of their nests baked by the sun. I've observed mounds over 30cm high. Lasius niger avoids shady woodlands and bogs where it is replaced by Lasius platythorax, which makes its nest out of organic material, such as dead wood or peat. Figure 4 - Lasius niger nest built up due to being overgrown by vegetation Photo © Harry E. Clarke Biology 2 of 4 The Silver-studded Blue life cycle starts with the female laying eggs close to ant nests (Jordano, 1992) of either Lasius niger or L. alienus between June and August. When the eggs hatch in the spring (or in July if there is a very rare second brood), the larvae are picked up by worker ants and taken back to their nest. As larvae develop, they are attended by the ants. The larvae secrete copious amounts of sugars and proteins which the ants feed upon. If it was not for the ants constantly keeping the Silver-studded Blue larvae clean, the larvae would probably succumb to fungal infections. The larvae feed at night, with the ants making excellent body guards for the larvae. The host ants have been shown to successfully defend the larvae from attack from other ant species (Jordano, 1992). When the Silver-studded Blue larva has completed its development, it pupates in or near the ant nests. It is possible that the pupae are able to communicate with the ants, as has been demonstrated in the Large Blue (Barbero, 2012). When the adult ultimately ecloses, ants remain in attendance while the butterfly expands and dries its wings. The adult's furry body is wet with droplets of liquid which are highly attractive to the ants (Thomas, 1985; Thomas, 2014). Males patrol areas containing ant nests, searching for freshly eclosed females with which to mate. Experiments in Spain showed that displaced males soon return to these areas (Seymour, 2003). Figure 5 - Silver-studded Blue copulating pair Photo © Harry E. Clarke Females mate shortly after emergence (Thomas, 1985). The copulating pairs I have observed are always of females with fully expanded wings and with no ants present. The ants possibly prevent males from mating too soon. Once the females have mated, they then hide for a few days to allow their eggs to mature (Seymour, 2003), and to avoid the attention of males. Hence they will emigrate from the immediate vicinity of the natal patch. When the eggs are mature, the female will then search out nests of their host ant species for oviposition. Displacement In a study in the Doñana National Park in southern Spain, researchers recorded a movement of 900 m for Silver-studded Blue from where it was released in a fragmented habitat, and 783 m in a less fragmented area. They moved males 200 m away from Lasius niger nests, and within a day or so, all had returned to the capture area (Seymour, 2003). I have observed a male zoom around an area of 15 to 20 m diameter at Hankley Common in Surrey. In under a minute it must have covered over 100 m, the area being typical Silver-studded Blue habitat. I have observed a female Silver-studded Blue at least 500 m from any suitable habitat. Whilst out surveying for Silver-studded Blues at Frensham Common in Surrey, I was in the middle of some tall Ling, when a small brown butterfly flew past me and landed a few metres away, where I was able to identify it as a female Silver-studded Blue. I was at least 500 m away from the nearest colony, which I had just come from, and it must have been at least 500 m to the next colony. In recent years in Surrey, there have been two examples of Silver-studded Blue colonising sites at least 3 km away from the nearest known colonies, with all the evidence pointing to a natural colonisation rather than an introduction. A small heathland (at SU9638) had recently been cleared, where a tiny colony, consisting of only a few Silver-studded Blues, was discovered in 2016. Given that the site was unknown by Lepidoperists (there were no records in the database), it is unlikely that anyone had released butterflies there (and certainly not the site owner). The colony lasted a couple of years, before dying out as the habitat became unsuitable. The nearest colony was 3 km away as the crow flies, with roads and woods inbetween. At another site (at SU8945) which had been recently cleared by the Amphibian and Reptile Trust, the Silver-studded Blue was discovered.

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